1. Field of the Invention
The present invention relates generally to secondary power sources and, more particularly, is concerned with an AC and DC motor-generator set employing modified annular end plates for improved water cooling of the windings of high power density AC and DC stators in the motor-generator set.
2. Description of the Prior Art
A secondary power source is typically provided in a nuclear submarine to generate reserve power. As the primary power source of the submarine, its nuclear power plant is operated to power the submarine; it also powers the secondary power source to charge DC batteries. Then, during periods when the nuclear plant is shut down or knocked off-line, the reserve power of the DC batteries is used to restart the primary source via the secondary source to operate the submarine.
One typical secondary power source used heretofore employs an AC and DC motor-generator set in which the AC and DC components are disposed in tandem arrangement on and along a common shaft. During periods of nuclear power plant operation, the AC component functions as a motor to rotatably drive the common shaft and thereby run the DC component as a generator to charge the DC batteries. Then, during periods when the nuclear power plant is shut down, the DC batteries operate the DC component as a motor to rotatably drive the common shaft and thereby run the AC component as a generator to operate the submarine.
Historically, the AC and DC components were cooled by air flow about the peripheries thereof. However, air cooled systems have many shortcomings. They are large and heavy and are noisy due to vane turbulence of their fans. Because of higher power density demands and the need to conserve space and weight in a submarine as well as for the submarine to operate quietly, a primary design objective is to minimize the size and weight of components of the motor-generator set.
The cooling of the motor-generator set primarily by using water, as opposed to air, has been recently adopted as a better way to cool the motor-generator set. Water cooling offers greater opportunities to further reduce component size and weight and to make the set operate more quietly. U.S. Pat. No. 4,728,840 to Newhouse, assigned to the assignee of the present invention, discloses an improved arrangement for water cooling the AC and DC rotors of the motor-generator set.
Water cooling also has been adopted heretofore for cooling the cores of the AC and DC stators of high power density motor-generator sets. The cores of these stators are constructed of a plurality of flat annular plates (also known as "punchings") which when assembled or stacked together are used to support the stator windings. These stator cores also include an annular end plate (also known as a "fingerplate") disposed at each opposite end of the stack of support plates and a plurality of axially-extending attachment members (also known as "building bars") which are circumferentially spaced about the stack of support plates. The attachment members are installed in axial slots in the periphery of the stacked support plates and are rigidly fixed, such as welding, to the support plates and the end plates in order to retain the stack of support plates together.
The required power density of AC and DC stators is such that cooling the stator cores with forced air is not adequate for obtaining winding temperatures compatible with the electrical insulation materials being used in the motor-generator set. The addition of a cooling water system as the primary stator winding cooling means is required. Several approaches to water cooling have been proposed in the prior art.
One approach is to provide water cooling via passages running directly in the stator core windings. However, this approach has been rejected because it necessitates a network of cooling water passages that would be too complex. Further, the brazed joints necessary for electrically connecting the windings would have insufficient reliability (would have too great a potential for producing water leaks).
Another approach that has been adopted is to cool stator cores by incorporation of metal cooling water jacket assemblies, such as of stainless steel, shrunk onto the outside diameter of the stator cores. The jacket assemblies contain passages through which the ship's cooling water is circulated to remove heat generated in the stator windings and core during operation. The passages in the cooling water jacket assembly are obtained by welding an outer jacket cylinder onto an inner jacket cylinder having a labyrinth passageway formed on its outer surface that faces an inner surface of the outer jacket cylinder.
However, even though the cooling water jacket assembly approach is better than incorporating water flow passages directly in the stator windings, it too has several drawbacks which contribute to an overall lack of robustness of this adopted approach. First, the jacket assemblies which are not required for providing structural support for the respective stators do add considerable weight, amounting to approximately ten percent of the total motor-generator set weight.
Second, the motor-generator set stators axially are very short relative to their diameters. As a consequence, only about one-third of the total winding length is in contact with the stator cores, with the remaining two-thirds of the winding length making up end turns. Therefore, the thermal resistance between the ends of the winding end turns and the stator cooling water in the jacket assemblies is relatively high due to the long conduction path. The maximum surface temperature of the windings is located at the ends of the winding end turns which are the farthest locations from the cooling water.
Third, the convective film coefficient of water in the water cooling jacket assemblies must be maintained as high as possible to minimize the maximum winding surface temperature; however, the pressure drop allowed for the ship's water in the motor-generator set is very small, thereby significantly limiting water velocities and convective film coefficients. This results in the necessity of the complex water passage labyrinth design to minimize pressure drop and maximize film coefficients in the AC and DC stator cooling water jacket passages.
Fourth, even with the use of water in the cooling water jacket assemblies for cooling the stator windings and cores, some forced air is still needed for cooling the stator winding end turns to limit the maximum winding surface temperature to acceptable limits. This requires considerable free space on both sides of the winding end turns (inside diameter and outside diameter) for adequate air circulation. This requirement precludes significant support of the winding end turns during shock loads.
Consequently, a need still exists for further improvement of approaches to water cooling of AC and DC stator cores and windings in the motor-generator set.